if I have a step up transformer how do i estimate the inductance required so that when the secondry is shorted it dose not damage my primary.??

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I'm not being flip, but are you sure that such a thing is even possible? I don't know what kind of components you would use in such a circuit since I've never seen a component with a rating that high. A vacuum tube might exist with 40kV on the plate but no semiconductor I've ever seen would sustain that much voltage.

The folks who make paint spray booths for car bodys have a thing called a high tension cascade where they start with about 10kV and go through a series of what I believe are capacitative voltage doublers or triplers. They use this to charge up the paint particles just before they launch them in the general direction of a grounded car body.

Practically speaking, the inductance of the secondary is going to be determined by your circuit requierment. That wll be the switching frequency and the current draw.

You do short circuit protection by means of a crontrol circuit on the secondary side that senses current and will cause the primary commutation circuit to shut down.

Even assuming you can find a frequency to drive this monster - thinking of the step-up ratio - it's gonna be interesting. That 40 KV is really going to want to reach out and touch somebody. Hope can run the thing in oil.

What kind of current are you intending to produce? I've been around an old X-ray that ran 60 KV at 60 mills. All voltages that high tend to produce X-rays as well as arcs. I'd rather try tiger roping.

Please be extremely careful. Insulators just don't behave the same way at those voltages. Be very aware of corona as well.

If you've already worked with these voltages, then you know this already and may assume that I am repeating it for the benifit of onlookers. If not, then please learn about the hazards. Always take appropriate safety precautions!

I am looking at designing PWM based High voltage SMPS around maximum 40 KV .

dose sombody have any idea ,where to start.

if I have a step up transformer how do i estimate the inductance required so that when the secondry is shorted it dose not damage my primary.??

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Most modern Dental X-ray machines already do exactly what you are trying to design. A switch-mode type of power supply (may or may not be PWM, depending on the make and model) resides in the control box that mounts on the wall, and provides the relatively low voltage square wave that drives a step-up transformer inside the tubehead. Also inside the tubehead you would find a voltage multiplier circuit, typically a series of 6 to 8 hv diodes and hv capacitors. The end result is a power supply that provides between 60KV to 70KV at 7mA for the X-ray tube. Most designs utilize voltage and current feedback for regulation and circuit protection.

You should be able to get hold of Service manuals, which include electrical schematic diagrams, from various Dental X-ray manufacturers such as Sirona, Gendex, Belmont, and others.